Etalon based spectrometers are well known devices for measuring the intensity of light in a beam as a function of wavelength. FIG. 1 shows the features of a prior art etalon spectrometer used for measurement of wavelength and bandwidth of a laser beam 16. The beam is diffused by diffuser 2 so that rays propagating in a very large number of angles illuminate etalon 4. FIG. 1 shows a single ray 20 being reflected many times within the etalon gap between surfaces 8A and 8B which are coated to reflect about 90%. Spectral components which are transmitted through the etalon are focused by lens 14 onto photo diode array 12. Photo diode array 12 registers a fringe pattern 15 which can be read using electronic data acquisition boards (not shown). The transmission or reflection of light incident on an etalon such as that depicted is well understood and depends on the design of the etalon, particularly the reflectance of the two reflecting surfaces.
Etalon spectrometers are widely used to measure the spectrum of lasers. A particularly important use of etalon spectrometers is to measure the bandwidth of line narrowed excimer lasers such as the line narrowed KrF excimer laser. These lasers are used, for example, as light sources for deep-UV microlithography. There are two spectral characteristics of these lasers which are very important for microlithography applications. These are the spectral bandwidth of the laser measured at 50 percent of the peak intensity, called its full width-half maximum band width (abbreviated xcex94xcexFWHM), and the spectral bandwidth, which contains 95% of laser energy called the 95% integral bandwidth (abbreviated xcex94xcex95%). It is very important that the laser is always operating within specifications during microlithography chip manufacturing because spectral broadening would cause blurring of the integrated circuits being printed on silicon wafers which will result in yield problems. Therefore, it is very important to provide continuous monitoring capabilities for the laser spectrum.
The prior art etalon spectrometer is capable of accurately measuring xcex94xcexFWHM values, and is currently used for this purpose in production microlithography lasers, such as manufactured by CYMER, Inc. (San Diego, Calif.). However, prior art etalon spectrometers are not very suitable for accurately measuring xcex94xcex95% values. Typical production quality KrF excimer lasers should have a xcex94xcexFWHM of about 0.6 pm and xcex94xcex95% of about 2 pm, if operating properly.
FIG. 2 shows the calculated slit function spectrum of a typical prior art etalon having a free spectral range (FSR) of 5 pm and a coefficient of finesse (finesse) of 38. (The terms FSR and finesse are defined and explained in a variety of optic texts such as OPTICS by Eugene Hecht/Alfred Zajae published by Addison-Wesley, Reading, Mass.) The slit function spectrum of FIG. 2 can be derived from one of the peaks of fringe pattern 15. The calculation graphed in FIG. 2 assumes that the light illuminating the etalon is monochomatic (i.e., an infinitely narrow bandwidth). If such an etalon is used to measure the bandwidth of a laser beam, the slit function bandwidth of the etalon is a source of error and contributes to uncertainty or error in the measurement. The calculated FWHM bandwidth for this prior art etalon is 0.13 pm and the 95% integral bandwidth for the etalon is about 1.5 pm.
For the etalon to accurately measure spectrum of a real laser, the slit function bandwidth of the etalon itself should be substantially smaller than the laser bandwidth. While this condition is satisfied for xcex94xcexFWHM measurements, where etalon slit function FWHM of 0.13 pm is substantially smaller than typical laser xcex94xcexFWHM of about 0.6 pm, the same is not true for xcex94xcex95% measurements, where etalon slit function bandwidth of about 1.5 pm is a substantial fraction of the expected laser bandwidth of about 2 pm.
Therefore, if the prior art etalon spectrometer with the FIG. 2 slit function is used to measure xcex94xcex95%, a complicated numerical analysis is needed to deconvolve the real xcex94xcex95% value. Such analysis is prone to errors and ambiguous results, so no reliable xcex94xcex95% information is available during the microlithography process. As a result, a laser can go out of specification unnoticed. This can lead to very expensive yield problems and should be avoided.
Another way of accurately measuring laser spectrum is to use a high resolution grating spectrometers. These instruments can provide accurate spectral measurement including accurate xcex94xcex95% measurements, but are very bulky and expensive. These instruments are successfully used in the laboratory but are not well suited for production line microlithography use.
What is needed is a compact spectrometer, capable of accurate measurement of both xcex94xcexFWHM and xcex94xcex95%, which can be built as a part of internal laser diagnostic set, so that it can be used in the field during the microlithography process.
The present invention provides a first double pass etalon based spectrometer. In a preferred embodiment a second etalon matched to the first double pass etalon is used to produce extremely precise fringe data. Spectral components of a diffused beam are angularly separated as they are transmitted through an etalon. A retroreflector reflects the transmitted components back through the etalon. Twice transmitted spectral components are directed through a second etalon and focused onto a light detector which in a preferred embodiment is a photo diode array. The spectrometer is very compact producing the extremely precise fringe data permitting bandwidth measurements with precision needed for microlithography for both xcex94xcexFWHM and xcex94xcex95%.